Control circuit and method for programming an output voltage of a power converter

ABSTRACT

A power converter determines a feedback signal according to a voltage signal related to an output voltage of the power converter and a reference voltage, thereby regulating the output voltage. A control circuit and method for programming the output voltage of the power converter utilize an offset current generator to inject a current or sink a current for changing the voltage signal or the reference signal, thereby adjusting the output voltage. As a result, it gets rid of complicated circuitry but provides more steps adjustment, which reduces related costs.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of co-pending application Ser. No.14/920,078, filed on Oct. 22, 2015, for which priority is claimed under35 U.S.C. § 120; and this application claims priority of Application No.104102151 filed in Taiwan on Jan. 22, 2015 under 35 U.S.C. § 119; theentire contents of all of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention is related generally to programmable powerconverters. More particularly, the present invention is related to acontrol circuit and method for programming an output voltage of a powerconverter.

BACKGROUND OF THE INVENTION

One of the charging methods for the portable electronic devices, forexample the smart phones and the tablet personal computers, is toutilize a power converter to convert the power into the output voltage.The output voltage is provided to the electronic devices for chargingthrough the transmission line connected to the power converter. FIG. 1shows a conventional power converter 2, which includes a transformer TXgenerated an output voltage V_(OUT) at a secondary side of thetransformer TX. Resistors R1 and R2 form a voltage divider which iscoupled to an output terminal of the power converter 2 to detect theoutput voltage V_(OUT) and to divide the output voltage V_(OUT) togenerate a voltage signal V_(DIV). A shunt regulator 4 is coupled to theresistors R1 and R2 as well as an optical coupler 6. The shunt regulator4 will compare the voltage signal V_(DIV) with a reference voltage todetermine the current Is flowing through the optical coupler 6.Accordingly, the optical coupler 6 generates a feedback signal Ifbrelated to the current Is for a control integrated circuit (not shown inFIG. 1) to regulate the output voltage V_(OUT). There are more and moresystems that need to change the output voltage V_(OUT) according todifferent needs, for example the rapid charging mode or the sleep mode,in order to improve their performance. There are two existing methodsfor changing the feedback signal Ifb in order to change the outputvoltage V_(OUT): changing a divider ratio of the voltage divider toadjust the voltage signal V_(DIV), or choosing a different referencevoltage.

As disclosed by U.S. Pat. No. 5,773,963, one of the methods for changinga divider ratio to adjust the output voltage of a power converter isillustrated in FIG. 2, in which a power circuit 8 converts a power inputV_(AC) into an output voltage V_(OUT) for charging a battery 10. Theoutput voltage V_(OUT) is divided by a divider ratio to generate avoltage signal V_(DIV) that is compared with a reference voltage V_(ref)to generate a feedback signal S_(FB) for the power circuit 8 to adjustthe output voltage V_(OUT). A microcontroller 12 controls the switchingof a transistor Q1 so as to determine whether a resistor R3 and theresistor R2 should be in a parallel connection. Accordingly, the dividerratio will be controlled and thereby adjust the output voltage V_(OUT).

On the other hand, U.S. Pat. No. 7,242,339 discloses a method forchoosing a reference voltage to adjust the output voltage of a powerconverter, as shown in FIG. 3, in which a resistor switch circuit 14 isutilized to choose a needed reference Vref in a way that switches Q2 toQ7 in the resistor switch circuit 14 are selectively turned on to seteither one of different voltages as the voltage Vp to be divided byseveral serially-connected resistors, and channel gates Gate 1 to Gate 5are controlled to choose one of the divided voltage as the referencevoltage Vref.

The existing methods for changing the output voltage mainly utilize theswitch and the resistor to change the output voltage V_(OUT). If theadjustable steps of the output voltage V_(OUT) are to be raised, thenumbers of the resistor and the switch have to be increased. Adversely,related costs are also increased. Moreover, these switches and resistorscan be merely installed out of the integrated circuit (IC), whichresults in a complicate circuitry.

SUMMARY OF THE INVENTION

An objective of the present invention is to provide a control circuitand method for programming an output voltage of a power converter.

According to the present invention, a control circuit for programming anoutput voltage of a power converter, which includes a feedback loopconfigured to operably provide a feedback signal related to the outputvoltage for regulating the output voltage, comprises a voltage dividerand an offset current generator. The voltage divider divides the outputvoltage to generate a voltage signal for controlling the feedbacksignal. The offset current generator is coupled to the voltage dividerfor injecting a first current to the voltage divider or sinking a secondcurrent from the voltage divider, thereby adjusting the voltage signaland the output voltage.

According to the present invention, a control circuit for programming anoutput voltage of a power converter, which includes a feedback loopconfigured to operably provide a feedback signal related to the outputvoltage for regulating the output voltage, comprises a voltage divider,an offset resistor, and an offset current generator. The voltage dividerdivides the output voltage to generate a first voltage signal. Theoffset resistor has a first terminal coupled to the voltage divider toreceive the first voltage signal, and a second terminal to provide asecond voltage signal for controlling the feedback signal. The offsetcurrent generator is coupled to the second terminal of the offsetresistor for injecting a first current to the offset resistor or sinkinga second current from the offset resistor, thereby offsetting the firstvoltage signal to generate the second voltage signal. The offset currentgenerator adjusts the first current or the second current to adjust thesecond voltage so that the feedback signal will be changed so as toadjust the output voltage.

According to the present invention, a control circuit for programming anoutput voltage of a power converter, which includes a feedback loopconfigured to operably provide a feedback signal related to the outputvoltage for regulating the output voltage at a target level, comprises afirst digital to analog converter, an offset resistor, and an offsetcurrent generator. The first digital to analog converter provides afirst reference voltage. The offset resistor has a first terminalcoupled to the first digital to analog converter to receive the firstreference voltage, and a second terminal to provide a second referencevoltage for determining the target level. The offset current generatoris coupled to the second terminal of the offset resistor for injecting afirst current to the offset resistor or sinking a second current fromthe offset resistor, thereby offsetting the first reference voltage togenerate the second reference voltage. The offset current generatoradjusts the first current or the second current to adjust the secondreference voltage, thereby adjusting the target level.

According to the present invention, a control circuit for programming anoutput voltage of a power converter, which includes a transformer, anauxiliary coil in a primary side of the transformer and configured tooperably detect the output voltage to generate a first voltage signal,and a voltage divider coupled to the auxiliary coil to divide the firstvoltage signal to generate a second voltage signal, comprises an offsetresistor and an offset current generator. The offset resistor has afirst terminal coupled to the voltage divider to receive the secondvoltage signal, and a second terminal to provide a third voltage signal.The offset current generator is coupled to the second terminal of theoffset resistor for injecting a first current to the offset resistor orsinking a second current from the offset resistor, thereby offsettingthe second voltage signal to generate the third voltage signal so as toregulate the output voltage. The offset current generator adjusts thefirst current or the second current to adjust the third voltage signal,thereby adjusting the output voltage.

According to the present invention, a control method for programming anoutput voltage of a power converter, which includes a feedback loopconfigured to operably provide a feedback signal related to the outputvoltage so as to regulate the output voltage, comprises the steps of:dividing the output voltage by a voltage divider to generate a voltagesignal so as to control the feedback signal, and injecting a firstcurrent to the voltage divider or sinking a second current from thevoltage divider so as to change the voltage signal to adjust the outputvoltage.

According to the present invention, a control method for programming anoutput voltage of a power converter, which includes a feedback loopconfigured to operably provide a feedback signal related to the outputvoltage so as to regulate the output voltage, comprises the steps of:dividing the output voltage to generate a first voltage signal appliedto a first terminal of an offset resistor, injecting a first current toa second terminal of the offset resistor or sinking a second currentfrom the second terminal of the offset resistor, thereby offsetting thefirst voltage signal to generate a second voltage signal so as tocontrol the feedback signal, and adjusting the first current or thesecond current so as to adjust the second voltage signal, therebychanging the feedback signal to adjust the output voltage.

According to the present invention, a control method for programming anoutput voltage of a power converter, which includes a feedback loopconfigured to operably provide a feedback signal related to the outputvoltage so as to regulate the output voltage at a target level,comprises the steps of: providing a first reference voltage to a firstterminal of an offset resistor, injecting a first current to a secondterminal of the offset resistor or sinking a second current from thesecond terminal of the offset resistor, thereby offsetting the firstreference voltage to generate a second reference voltage so as todetermine the target level, and adjusting the first current or thesecond current so as to adjust the second reference voltage, therebyadjusting the target level.

According to the present invention, a control method for programming anoutput voltage of a power converter, which includes a transformer, andan auxiliary coil in a primary side of the transformer and configured tooperably detect the output voltage to generate a first voltage signal,comprises the steps of: dividing the first voltage signal to generate asecond voltage signal applied to a first terminal of an offset resistor,injecting a first current to a second terminal of the offset resistor orsinking a second current from the second terminal of the offsetresistor, thereby offsetting the second voltage signal to generate athird reference voltage so as to regulate the output voltage, andadjusting the first current or the second current so as to regulate thethird voltage signal to adjust the output voltage.

In contrast with the conventional methods for changing the divider ratioand choosing the reference voltage, control circuits and methodsaccording to the present invention, which adjust the output voltage of apower converter via controlling the first and the second currents, aresimpler. Particularly, the present invention needs no extra switches orresistors to increase the adjustable steps.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objectives, features and advantages of the presentinvention will become apparent to those skilled in the art uponconsideration of the following description of the preferred embodimentsaccording to the present invention taken in conjunction with theaccompanying drawings, in which:

FIG. 1 shows a conventional power converter;

FIG. 2 shows a conventional method for changing a divider ratio of avoltage divider;

FIG. 3 shows a conventional method for choosing a reference voltage;

FIG. 4 shows a first embodiment of the present invention;

FIG. 5 is a simplified circuitry of FIG. 4;

FIG. 6 shows a second embodiment of the present invention;

FIG. 7 shows a simplified circuitry of FIG. 6;

FIG. 8 shows a third embodiment of the present invention; and

FIG. 9 shows a fourth embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 4 shows a first embodiment of the present invention, and FIG. 5shows a simplified circuitry of FIG. 4. Referring to FIGS. 4 and 5, at asecondary side of a transformer TX of a power converter 2, an opticalcoupler 6 is served as a feedback loop for providing a feedback signalIfb related to the output voltage V_(OUT) to regulate the output voltageV_(OUT). A control circuit 16 is coupled to the optical coupler 6 forcontrolling the feedback signal Ifb so as to regulate the output voltageV_(OUT). Alternatively, the optical coupler 6 can be replaced by otherfeedback loop.

The control circuit 16 includes a voltage divider 18 that is formed bythe series resistors R1 and R2 for dividing the output voltage V_(OUT)to generate a voltage signal V_(DIV) for controlling the feedback signalIfb. An offset current generator 20 has a terminal 22 coupled to thevoltage divider 18 and a terminal 24 coupled to a microcontroller 26.The offset current generator 20 injects a current I1 to the voltagedivider 18 or sinks a current I2 from the voltage divider 18, therebyadjusting the voltage signal V_(DIV) so as to adjust the output voltageV_(OUT). In the offset current generator 20, a decoder 28 decodes adigital signal S_(D1) to generate control signals S_(C1) and S_(C2) aswell as switch signals S_(S1) and S_(S2). A digital to analog converter30 is coupled to the decoder 28 and the voltage divider 18. The digitalto analog converter (DAC) 30 includes switches SW1 and SW2 and variablecurrent sources 32 and 34. The switch SW1 is coupled between thevariable current source 32 and the voltage divider 18. The switch SW2 iscoupled between the voltage divider 18 and the variable current source34. The variable current source 32 adjusts the amount of the current I1according to the control signal S_(C1), and the variable current source34 adjusts the amount of the current I2 according to the control signalS_(C2). The switches SW1 and SW2 will be turned on or turned offaccording to the switch signals S_(S1) and S_(S2). When the switchsignal S_(S1) turns on the switch SW1, the variable current source 32injects the current I1 into the resistor R2 in the voltage divider 18.When the switch signal S_(S2) turns on the switch SW2, the variablecurrent source 34 sinks the current 12 from the resistor R2 in thevoltage divider 18.

The control circuit 16 shown in FIG. 4 further includes amicrocontroller 26 that includes a program memory 46 and a data memory48 for saving information. The microcontroller 26 generates the digitalsignals S_(D1) and S_(D2) according to the saved information to thedecoder 28 and a digital to analog converter 36, respectively. Accordingto the digital signal S_(D2), the digital to analog converter 36generates a reference voltage Vref at an output terminal 38 andgenerates an overvoltage threshold Vref_ov at an output terminal 40. Anerror amplifier 42 has a positive input terminal coupled to the voltagedivider 18 and the digital to analog converter 30, a negative inputterminal coupled to the output terminal 38 of the digital to analogconverter 36, and an output terminal connected to a control terminal 50of a transistor Q8. The transistor Q8 is coupled between the opticalcoupler 6 and a ground terminal. The error amplifier 42 controls thecurrent Is flowing through the transistor Q8 according to a differencebetween the voltage signal V_(DIV) and the reference voltage Vref. Theoptical coupler 6 generates the feedback signal Ifb according to thecurrent Is. A shunt regulator 4 in FIG. 5 is formed by the transistor Q8and the error amplifier 42. In this embodiment, a comparator 44 isutilized for achieving an overvoltage protection, in which a positiveinput terminal of the comparator 44 is coupled to the voltage divider18, and a negative input terminal of the comparator 44 is coupled to theoutput terminal 40 of the digital to analog converter 36. When thevoltage signal V_(DIV) is higher than the overvoltage threshold Vref_ov,the comparator 44 generates a protecting signal Sov to turn off thepower converter 2 so as to achieve the overvoltage protection. In otherembodiments, the comparator 44 can be also utilized for achieving anunder voltage protection. In such circumstances, the digital to analogconverter 36 will provide an under voltage threshold supplied to thepositive terminal of the comparator 44, and the negative input terminalof the comparator 44 will receive the voltage signal V_(DIV). When thevoltage signal V_(DIV) is lower than the under voltage threshold, thecomparator 44 will generate a protecting signal to turn off the powerconverter 2, thereby achieving the under voltage protection.

In FIG. 4, when the offset current generator 20 neither injects thecurrent I1 nor sinks the current I2, the output voltage isV _(OUT) =Vref×(R1+R2)/R2.   (EQ-1)

When the offset current generator 20 injects the current I1 into thevoltage divider, the output voltage isV _(OUT) =Vref×(R1+R2)/R2−I1×R1.   (EQ-2)

When the offset current generator 20 sinks the current I2 f from thevoltage divider, the output voltage isV_(OUT) =Vref×(R1+R2)/R2+I2×R1.   (EQ-3)

As illustrated by the equations EQ-2 and EQ-3, the offset currentgenerator 20 can adjust the output voltage V_(OUT) by adjusting thecurrent I1 or I2. The adjustable steps of the output voltage V_(OUT) canbe increased as long as the decoder 28 with a higher resolution (i.e.bit numbers) and the digital to analog converter 30 with a higherresolution are selected. No extra components (resistors or switches) areneeded. Moreover, as shown in FIG. 5, the offset current generator 20,the microcontroller 26, and the digital to analog converter 36 can beintegrated in an IC in order to simplify the circuitry of the powerconverter 2. The resolutions of the digital to analog converters 30 and36 can be determined according to requirements. The resolution of thedigital to analog converter 30 can be lower than that of the digital toanalog converter 36.

FIG. 6 shows a second embodiment of the present invention, and FIG. 7shows a simplified circuitry of FIG. 6. In FIGS. 6 and 7, the opticalcoupler 6 serves as a feedback loop for providing the feedback signalIfb related to the output voltage V_(OUT) for regulating the outputvoltage V_(OUT). The control circuit 16 is coupled to the opticalcoupler 6 for controlling the feedback signal Ifb and adjusting theoutput voltage V_(OUT). Alternatively, the optical coupler 6 can bereplaced by other suitable feedback loop. The control circuit 16 in FIG.6 has the same offset current generator 20, microcontroller 26, anddigital to analog converter 36 as those in FIG. 4. In the controlcircuit 16 of FIG. 6, the voltage divider 18 formed by the seriesresistors R1 and R2 divides the output voltage V_(OUT) to generate thevoltage signal V_(DIV1). A first terminal of the offset resistor Ros iscoupled to the voltage divider 18, and a second terminal of the offsetresistor Ros is coupled to the terminal 22 of the offset currentgenerator 20. When the switch signal S_(S1) turns on the switch SW1 thatis coupled between the variable current source 32 and the secondterminal of the offset resistor Ros in the offset current generator 20,the variable current source 32 thence injects the current I1 to theoffset resistor Ros. When the switch signal S_(S2) turns on the switchSW2 coupled between the second terminal of the offset resistor Ros andthe variable current source 34 in the offset current generator 20, thevariable current source 34 sinks current I2 from the offset resistorRos. The offset resistor Ros generates an offset voltage in accordancewith the current I1 or I2, thereby offsetting the voltage signalV_(DIV1) so as to generate the voltage signal V_(DIV2) at the secondterminal of the offset resistor Ros.

An error amplifier 42 of the control circuit 16 in FIG. 6 has thepositive input terminal coupled to the offset resistor Ros and thedigital to analog converter 30, the negative input terminal coupled tothe output terminal 38 of the digital to analog converter 36, and theoutput terminal coupled to the control terminal 50 of the transistor Q8.The transistor Q8 is coupled between the optical coupler 6 and theground terminal. The error amplifier 40 controls the current Is that ispassing through the transistor Q8 according to the difference betweenthe voltage signal V_(DIV2) and the reference voltage Vref. The opticalcoupler 6 generates the feedback signal Ifb according to the current Is.The variable current sources 32 and 34 of the offset current generator20 respectively control the amounts of the currents I1 and I2 accordingto the control signals Sc1 and Sc2, thereby controlling the voltagesignal V_(DIV2) so as to adjust the feedback signal Ifb to adjust theoutput voltage V_(OUT). In the control circuit 16 in FIG. 6, thepositive terminal of the comparator 44 is coupled to the offset resistorRos, the negative terminal thereof is coupled to the output terminal 40of the digital to analog converter 36. The comparator 44 generates theprotecting signal Sov when the voltage signal V_(DIV2) is higher thanthe overvoltage threshold Vref_ov for turning off the power converter 2so as to achieve the overvoltage protection. In other embodiments, thecomparator 44 can be applied to the under voltage protection. In suchcircumstances, the positive terminal of the comparator receives theunder voltage threshold provided by the digital to analog converter 36,and the negative terminal of the comparator receives the voltage signalV_(DIV2). When the voltage signal V_(DIV2) is lower than the undervoltage threshold, a protecting signal will be generated, therebyturning off the power converter 2 to achieve the under voltageprotection.

In FIG. 6, supposed that the resistance value of the offset resistor Rosis much higher than the resistors R1 and R2, when the current I1 isinjected to the offset resistor Ros, the output voltage isV _(OUT)=(Vref−I1×Ros)×(R1+R2)/R2.   (EQ-4)

When the current I2 is sunken from the offset resistor Ros, the outputvoltage isV _(OUT)=(Vref+I2×Ros)×(R1+R2)/R2.   (EQ-5)

As illustrated by the equations EQ-4 and EQ-5, the control circuit 16 inFIG. 6 is able to adjust the output voltage V_(OUT) via adjusting thecurrents I1 or I2. The adjustable steps of the output voltage V_(OUT)can be increased as long as the decoder 28 with a higher resolution(i.e. bit numbers) and the digital to analog converter 30 with a higherresolution are selected. No extra components (resistors or switches) areneeded. Moreover, as shown in FIG. 7, the offset current generator 20,the microcontroller 26, and the digital to analog converter 36 can beintegrated in the IC 52 in order to simplify the circuitry of the powerconverter 2. The resolutions of the digital to analog converters 30 and36 can be determined according to requirements. The resolution of thedigital to analog converter 30 can be lower than that of the digital toanalog converter 36.

FIG. 8 shows a third embodiment of the present invention, whichcomprises the same voltage divider 18, offset current generator 20,microcontroller 26, and digital to analog converter 36 as those in FIG.4. In the circuitry of FIG. 8, the optical coupler 6 serving as thefeedback loop is also set at the secondary side of the transformer TX(not shown in FIG. 8) of the power converter 2 for providing thefeedback signal Ifb related to the output voltage V_(OUT) so as toregulating the output voltage V_(OUT) at a target level. In thisembodiment, the voltage divider 18 divides the output voltage V_(OUT) togenerate the voltage signal V_(DIV) related to the output voltage, theoutput terminal 38 of the digital to analog converter 36 provides thereference voltage Vref1, the first terminal of the offset resistor Rosis coupled to the output terminal 38 of the digital to analog converter36 for receiving the reference voltage Vref1, the second terminal of theoffset resistor Ros is coupled to the terminal 22 of the offset currentgenerator 20 and outputs the reference voltage Vref2 for determining thetarget level of the output voltage V_(OUT), the offset current generator20 injects the current I1 to the offset resistor Ros or sinks thecurrent I2 from the offset resistor Ros, and the error amplifier 42 hasa positive input terminal coupled to the voltage divider 18, a negativeinput terminal coupled to the digital to analog converter 30 and theoffset resistor Ros, and an output terminal coupled to the controlterminal 50 of the transistor Q8 that is coupled between the opticalcoupler 6 and a ground terminal. The error amplifier 42 controls thecurrent Is flowing through the transistor Q8 according to a differencebetween the voltage signal V_(DIV) and the reference voltage Vref2. Theoptical coupler 6 determines the feedback signal Ifb according to thecurrent Is. When the switch signal S_(S1) turns on the switch SW1 of theoffset current generator 20, the variable current source 32 determinesthe current I1 that is injected into the offset resistor Ros accordingto the control signal S_(C1). When the switch signal S_(S2) turns on theswitch SW2 of the offset current generator 20, the variable currentsource 34 determines the current I2 that is sunken from the offsetresistor Ros according to the control signal S_(C2). Accordingly, theoffset current generator 20 can change the offset voltage of the offsetresistor Ros by adjusting the currents I1 and I2, thereby determiningthe reference voltage Vref2 to change the target level of the outputvoltage V_(OUT). The conventional complicated circuitry for choosing thereference voltage Vref2 is not required. The adjustable steps of theoutput voltage V_(OUT) can be increased as long as the decoder 28 with ahigher resolution (i.e. bit numbers) and the digital to analog converter30 with a higher resolution are selected. No extra components (resistorsor switches) are needed. Moreover, the offset current generator 20, themicrocontroller 26, and the digital to analog converter 36 can beintegrated in the IC in order to simplify the circuitry of the powerconverter 2. The resolutions of the digital to analog converters 30 and36 can be determined according to requirements. The resolution of thedigital to analog converter 30 can be lower than that of the digital toanalog converter 36.

The control circuit 16 in FIG. 8 also includes the comparator 44. Thepositive terminal of the comparator 44 is coupled to the voltage divider18 for receiving the voltage signal V_(DIV). The negative terminal thecomparator 44 is coupled to the output terminal 40 of the digital toanalog converter 36. The comparator 44 generates the protecting signalSov when the voltage signal V_(DIV) is higher than the overvoltagethreshold Vref_ov provided by the digital to analog converter 36,thereby turning off the power converter 2 to achieve the overvoltageprotection. In other embodiments, the comparator 44 can be applied foran under voltage protection. In such circumstances, the positiveterminal of the comparator 44 receives the under voltage thresholdprovided by the digital to analog converter 36, and the negativeterminal thereof receives the voltage signal V_(DIV). When the voltagesignal V_(DIV) is lower than the under voltage threshold, the protectingsignal will be generated, thereby turning off the power converter 2 toachieve the under voltage protection.

The offset current generator 20 in FIGS. 4 to 8 is configured at thesecondary side of the transformer TX of the power converter 2. However,the offset current generator 20 can be configured at the primary side ofthe transformer TX, as illustrated in a fourth embodiment of the presentinvention as shown in FIG. 9. The power converter 2 includes atransformer TX converting an input voltage V_(IN) into the outputvoltage V_(OUT) by a primary side coil Np and a secondary side coil Ns.An auxiliary coil N_(FB) of the transformer TX is installed at theprimary side of the transformer TX for detecting the output voltageV_(OUT) to generate the voltage signal V_(FB). The serially-connectedresistors R1 and R2 form the voltage divider 18 coupled to the auxiliarycoil N_(FB) for dividing the voltage signal V_(FB) to generate thevoltage signal V_(DIV1). In the embodiment of FIG. 9, the controlcircuit of the present invention includes the offset resistor Ros andthe offset current generator 20. The first terminal of the offsetresistor Ros is coupled to the voltage divider 18 for receiving thevoltage signal V_(DIV1). The second terminal of the offset Ros iscoupled to a feedback terminal FB of the control IC 54 and provides thevoltage signal V_(DIV2). The control IC 54 controls the switching of thetransistor Q9 according to the voltage signal V_(DIV2), therebyregulating the output voltage V_(OUT). The offset current generator 20is integrated in the IC so as to simplify the circuitry of the powerconverter 2. The offset current generator 20 is coupled to the offsetresistor Ros via the terminal 22 and receives the digital signal S_(D1)via the terminal 24. The digital signal S_(D1) is provided by themicrocontroller 26 as shown in FIG. 4. In the offset current generator20, the decoder 28 decodes the digital signals S_(D1) to generatecontrol signals S_(C1) and S_(C2)as well as switch signals S_(S1) andS_(S2). The digital to analog converter 30 of the offset currentgenerator 20 includes variable current sources 32 and 34 and switchesSW1 and SW2. The switch SW1 is coupled between the variable currentsource 32 and the second terminal of the offset resistor Ros. The switchSW2 is coupled between the second terminal of the offset resistor Rosand the variable current source 34. The variable current source 32adjusts the current I1 that is injected into the offset resistor Rosaccording to the control signal S_(C1) when the switch signal S_(S1)turns on the switch SW1 of the offset current generator 20. The variablecurrent source 34 adjusts the current I2 that is sunken from the offsetresistor Ros according to the control signal S_(C2) when the switchsignal S_(S2) turns on the switch SW2 of the offset current generator20. The offset current generator 20 changes the offset voltage of theoffset resistor Ros by adjusting the currents I1 and I2, therebyadjusting the voltage signal V_(DIV2) so as to the output voltageV_(OUT). The adjustable steps of the output voltage V_(OUT) can beincreased as long as the decoder 28 with a higher resolution (i.e. bitnumbers) and the digital to analog converter 30 with a higher resolutionare selected. No extra components (resistors or switches) are needed.

In contrast with the conventional method for changing the divider ratioby changing the resistances and choosing the reference voltage, controlcircuits and methods according to the present invention, which controlthe current I1 or I2 to adjust the output voltage V_(OUT), are simpler.Moreover, the adjustable steps can be easily increased. The presentinvention can be applied to not only the AC to DC power converters butalso the DC to DC power converters, or other type of power converters.

While the present invention has been described in conjunction withpreferred embodiments thereof, it is evident that many alternatives,modifications and variations will be apparent to those skilled in theart. Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and scopethereof as set forth in the appended claims.

What is claimed is:
 1. A control circuit for programming an outputvoltage of a power converter including an auxiliary coil in a primaryside of a transformer configured to operably detect the output voltageto generate a first voltage signal and a voltage divider coupled to theauxiliary coil configured to operably divide the first voltage signal togenerate a second voltage signal, the control circuit comprising: anoffset resistor having a first terminal coupled to the voltage dividerconfigured to operably receive the second voltage signal, and a secondterminal configured to operably provide a third voltage signal; and anoffset current generator coupled to the second terminal of the offsetresistor and configured to operably inject a first current to the offsetresistor or sink a second current from the offset resistor so as tooffset the second voltage signal to generate the third voltage signal toregulate the output voltage; wherein, the offset current generatoradjusts the first current or the second current so as to adjust thethird voltage signal, thereby adjusting the output voltage.
 2. Thecontrol circuit of claim 1, wherein the offset current generatorcomprises: a decoder configured to operably decode a first digitalsignal; and a digital to analog converter, coupled to the decoder andthe second terminal of the offset resistor, configured to operablyinject the first current to the offset resistor or sink the secondcurrent from the offset resistor according to outputs of the decoder. 3.The control circuit of claim 2, wherein the outputs of the decodercomprising a first control signal, a second control signal, a firstswitch signal, and a second switch signal.
 4. The control circuit ofclaim 3, wherein the digital to analog converter comprises: a firstvariable current source configured to operably adjust the first currentaccording to the first control signal; a first switch coupled betweenthe first variable current source and the second terminal of the offsetresistor; wherein, the first switch will be turned on or turned offaccording to the first switch signal; a second variable current sourceconfigured to operably adjust the second current according to the secondcontrol signal; and a second switch coupled between the second variablecurrent source and the second terminal of the offset resistor; wherein,the second switch will be turned on or turned off according to thesecond switch signal.
 5. A control method for programming an outputvoltage of a power converter including an auxiliary coil in primary sideof a transformer configured to operably detect the output voltage togenerate a first voltage signal, the control method comprising the stepsof: dividing the first voltage signal to generate a second voltagesignal to a first terminal of an offset resistor; injecting a firstcurrent to a second terminal of the offset resistor or sinking a secondcurrent from the second terminal of the offset resistor, therebyoffsetting the second voltage signal to generate a third referencevoltage so as to regulate the output voltage; and adjusting the firstcurrent or the second current so as to regulate the third voltage signaland adjust the output voltage.